The present invention relates to portable hammer machines of the type comprising a housing with a cylinder therein, in which a reciprocating drive piston via a gas cushion in a working chamber repeatedly drives a hammer piston to impact on and to return from the neck of a tool carried by the machine housing as soon as a feeding force is applied via the machine housing to the tool and spring means interposed therebetween are compressed, the cylinder on the one hand being provided with primary ports for the passage of gas to and from the working chamber, which ports at impacting are opened above sealing means on the hammer piston to ventilate the working chamber, and on the other hand with secondary ports for ventilating the volume below the hammer piston during its reciprocation.
In prior embodiments these machines, of which one is described for example in patent GB 2,145,959, are subject to the inconvenience of the impact motor of the machine starting to pound as soon as the the tool is applied against the surface to be worked upon. That means that the initial collaring or pointing from the very first contact with the working surface has to be made under percussive action and, depending on the motor type often under full rotative motor speed, i.e. under full impact power, which makes it difficult to keep the tool exactly on the working spot aimed-at and also exposes the operator to injuries due to recoil and mis-directed blows.